The Sun's Travels and Why Fall is Nigh

Wednesday, Sept. 22 marks what scientists call the fall equinox in the Northern
Hemisphere, when night and day are of equal length and cooler weather is on
the way -- if it hasn't already arrived.

But what's behind the shift?

A year-long series of pictures by Greek photographer Anthony Ayiomamitis helps
put the equinox into perspective, while also illustrating why we have seasons.

The image shows the Sun's position in the sky at the same time each day, every
week or so, during a full year. The figure-8 pattern created by the Sun's apparent
annual movement is called an analemma.

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The Sun's apparent movement in the sky during all of 2003. The pattern
is an analemma.

? Anthony
Ayiomamitis

The high point comes during summer when the days are long, and the low point
in winter when the Sun remains frustratingly low on the horizon, days are short,
and less radiation is available to warm the ocean, air and land.

Ayiomamitis explains where the Sun is in his picture as we reach the fall equinox:

"The crossover point between the smaller upper loop and the larger lower loop
is Aug. 30," he explained. "We are now between the third and fourth solar disks
below and to the right of the crossover point."

The Sun appears to move around so much because Earth is tilted on its rotational
axis, by about 23.5 degrees. Earth's North Pole always points in roughly the
same direction in relation to the plane of the solar system. But as the planet
orbits the Sun (once each year) a major change occurs.

In the Northern Hemisphere's summer, the North Pole leans toward the Sun, causing
the Sun to be higher in our sky and spreading warmth copiously. In winter, the
pole leans away from the Sun and things get chilly (while those south of the
equator are treated to longer days). To visualize this, imagine pointing toward
someone who is at the center of a room, then walk around to the other side of
the room without changing your arm's pointing position in relation to your entire
city -- you'll then be pointing away from the person.

The analemma's figure-8 is not symmetrical. That's because Earth's 365-day
orbit is not a perfect circle. We are closer to the Sun in January, and this
causes Earth to move faster compared to summertime, making the bottom loop of
the analemma -- the winter portion -- larger.

(Being slightly closer to the Sun in winter does not provide enough extra warmth to overcome seasonal effects caused by the planet's tilt, however. More about that here.)

For his image, Ayiomamitis patiently made 47 exposures of the Sun between Jan.
7 and Dec. 20 last year, all on one frame of film. Then a second picture that
included the foreground -- the temple Hephaisteion, in Athens -- was taken and
added to the first.